Method of preparing thin magnetic films



1964 J. 5. MATHIAS ETAL 3,119,753

METHOD OF PREPARING THIN MAGNETIC FILMS Filed Oct. 5. 1960 H= 9o- 05750e --No HYPOPHOSPHI WITH HYPOPHOSPHITE H=O.57oe

30- l, H -O.380e

I I I H o lo I f I DRIVE IN OERSTEDS EASY DIRECTION EASY DIRECTION HARD DIRECTION HARD DIRECTION HYPIII IISQ ITE DRIVE FIELD DRIVE FIELD Io OERSTEDS HYPOPHOSPHITE IO OERSTEDS INVENTOR.

JOSEPH S. MATHIAS EDWIN F. SCHNEIDER ATTORNEX United States Patent Ofifice 3,119,753 Patented Jan. 28, 1964 3,119,753 METHOD GF PREPG THIN MAGNETIC FilLM Joseph S. Mathias, River-ton, Ni, and Edwin F. Schneider, .lenkintown, Pa, assignors to Sperry Rand Corporation, New York, N.Y., a corporation of Delaware Filed Oct. 5, 1960, er. No. 60,735 7 Claims. ((Il. 20443) This invention deals with an improved method of forming thin ferromagnetic films of nickel-iron alloys.

Thin ferromagnetic films of nickel-iron alloys are used extensively for computer information storage and in switching applications. in use, it is desirable that the films have relatively square hysteresis loops and low coercive forces. Various methods have been used, such as vacuum deposition of vapors of the metals and the like. It has also been proposed to prepare the thin films by electrodeposition from suitable plating solutions. The latter method has advantages, but the films produced often leave something to be desired in their properties.

The present invention constitutes an improved electroplating method for the formation of thin nickel-iron films. The most important characteristic of the present invention is the presence of the small amount of a hypophosphite. This permits a very precise control and results in films of lower coercivity, H and B and in the hard direction the hysteresis loop opens up to a much lower degree as will be shown in the drawings. An even more important property is switching time. The films of the present invention have much lower switching times under the same operating conditions than do films of the same character prepared without hypophosphite. The range of hypophosphite in grams per liter is from about 0.95 to 0.4. Optimum results are obtained with hypophosphite concentrations from 0.08 to 023 g./l.

While the most important single factor of the present invention is the presence of the small amount of hypophosphite, there are other conditions of the plating operation which are likewise important. The pH should be maintained in the range from 1.7 to 2.8, poorer results being obtained when the pH is outside of this range. Within the range of pHs, BH properties are excellent and are uniformly reproducible.

Another characteristic is current density. Optimum results are obtained at about 5 to 6 ma. At this current density, a film or" a thickness of approximately 880 A. can be deposited in 60 seconds.

Film thickness, while not as critical as hypophosphite content and pH, is, nevertheless, of importance. Good results are obtained with thicknesses from 800 to 1600' A.

The particular cation of the hypophosphite is not especially critical so long as it is not a platable metal. For practical operations sodium hypophosphite is preferred although, of course, the same results can be obtained with other alkali metal hypophosphites, such as potassium hypophosphite. However, there is no improvement and the higher cost makes it economically unattractive.

Except for the important factors of the present invention which have been set out above, the plating procedure is otherwise normal, and does not present any particular problems. This is an advantage of the present invention. As with most plating, the temperature is not especially critical, and room temperature may be used. There is no appreciable difference between the BH properties of films plated at room temperature and those plated at higher plating temperatures, for example in the range of 50 to 52 C. The relative insensitivity to small temperature changes makes control of the process of the present invention simple and is a desirable practical operating characteristic.

The nature of the substrate does not differ significantly from general plating practice by other methods. In general, a smooth conducting surface is desirable which, for example, can either be polished metal or glass metallized so that it has the necessary conductivity for electroplating. The latter type of substrate has some advantages and will be described in connection with specific examples, though, of course, the invention is not in any sense limited to the particular type of substrate used.

The proportions of nickel and iron in the final film can be varied over wide ranges by control of the various proportions of nickel and iron salts in the plating bath. Optimum results are obtained when the proportion of iron to nickel in the bath is between 1.5 to 98.5 to 3 to 97. This results in percentages of iron in the film from slightly below 14 to about 23%. It is an advantage of the present invention that the films produced have excellent properties over a considerable range of iron to nickel proportions so that in this respect no extremely critical control of plating bath composition is necessary.

It is well known that thin magnetic films can be produced with or without the presence of a strong magnetic field during plating. The same is true in the present invention. Even with a strong field of about 40% oersteds, there is no noticeable effect on the BH values though, of course, as is usual, the directional character of the anisotropy is more definitely fixed. The present invention, therefore, may be used with or without the presence of a magnetic field during plating. When an applied field is used, for purposes of anisotropy, the direction of application is such that the lines of force are disposed parallel to the plane of the plated film.

The invention will be described in greater detail in conjunction with the specific examples in which the parts are by weight unless otherwise specified and in connection with the drawings in which:

FIG. 1 is a series of curves of reciprocal switching times for two different films;

FIG. 2 is a pair of BH loops for a film prepared with hypophosphite, and

FIG. 3 is a similar pair for a film prepared without phosphite.

EXAMPLE 1 Plating baths were prepared having the following composition ranges:

Substrates were made from circular pieces of glass 9 mm. in diameter plated first with a film about a A. of chromium followed by 100 A. film of gold. Coating was by vacuum vapor deposition. The substrates were then plated in the bath in the conventional manner at room temperature using 4 to 6 volts and maintaining the current density at 6 ma./cm. pH of the various baths was varied from 1.5 to 3.4. Time of plating varied from approximately 60 seconds to about 2 minutes in order to produce films from 800 to 1600 A.

FIG. 1 shows curves of reciprocal switching times vs. drive for two films prepared as above, one with .3 g./l. hypophosphite and one without. It will be seen that the curves of the film prepared with the hypophosphite are much steeper than those of the other film. This permits faster switching times under any given set of drive conditions. This is of great importance in practical applications in computers and constitutes the most important advantage of films prepared by the process of the present invention.

FIGS. 2 and 3 illustrate hysteresis loops of the two films described above. It will be noted that in FIG. 2, which illustrates films prepared by the present invention, the loops are squarer and in the hard direction there is little or no opening of the loop. There is also a much lower coercive force shown in FIG. 2 which will be expressed in figures in following examples.

EXAMPLE 2 Plating was effected under conditions described in Example 1 in a series of baths having 0.3 g./l. of hypophosphite and varied amounts of ferrous sulfate. The following tabulation gives the composition of the films with change in ferrous sulfate concentration.

Grams Per Liter FeSO4.7H- O PercintflmFe in In general, there was a slight decrease in coercivity (H from 1.9 oersted for the 3.6 g./l. to 1.0 for 6 g./l. The increase in iron content also produced somewhat higher degrees of anisotropy.

EXAMPLE 3 A series of platings were made with fixed thickness, current density, bath composition and hypophosphite content. The pHs varied from 1.5 to 3.4. The composition of the bath, film thickness and resulting film properties are as follows:

EXAMPLE 4 A series of platings was efiected with different amounts of sodium hypophosphite. The plating data and characteristics were as follows:

It will be seen that the hypophosphite markedly reduces coercivity and improves other characteristics.

We claim:

1. In the method of producing relatively thin nickeliron-phosphorus films by electroplating, wherein the nickel-iron composition ranges from between about 77% and 86% nickel, balance iron, and wherein the method comprises plating on a conducting substrate from a plating bath comprising nickel and iron salts, the improvement comprising adding from 0.05 to 0.4 g./l. of an alkali metal hypophosphite to the bath and maintaining the pH of the plating bath below 3.

2. A method according to claim 1 in which the pH is from 1.7 to 2.8.

3. A method according to claim 2 in which the plating is efiected in a strong magnetic field with its lines of force parallel to the plane of the plated film.

4. A method according to claim 1 in which the plating is effected in a strong magnetic field with its lines of force parallel to the plane of the plated film.

5. A method according to claim 1 in which the alkali metal hypophosphite concentration is from 0.08 to 0.3 g./l. of hypophosphite.

6. A method according to claim 5 in which the plating is effected in a strong magnetic field with its lines of force parallel to the plane of the plated film.

7. A solution for use in electroplating nickel-iron magnetic films comprising approximately 218 g./l. of nickel sulfate, approximately 3.6-6.0 g./l. of ferrous sulfate and approximately 0.4 g./l. of sodium hypophosphite.

References Cited in the file of this patent 

1. IN THE METHOD OF PRODUCING RELATIVELY THIN NICKELIRON-PHOSPHRUS FILMS BY ELECTROPLATING, WHEREIN THE NICKEL-IRON COMPOSITION RANGES FROM BETWEEN ABOUT 77% AND 86% NICKEL, BALANCE IRON, AND WHEREIN THE METHOD COMPRISES PLATING ON A CONDUCTING SUBSTRTATE FROM A PLATING BATH COMPRISING NICKEL AND IRON SALTS, THE IMPROVEMENT COMPRISING ADDING FROM 0.35 TO 0.4 G./L. OF AN ALKALI METAL HYPOPHOSPHITE TO THE BATH AND MAINTAINGING THE PH OF THE PLANTING BATH BELOW
 3. 